x86/xen: Use intended values in HVM_CALLBACK_VECTOR

Replace the HVM_CB_GSI_GSI SHIFT and MASK with the existing
but currently unused HVM_CB_VECTOR_VECTOR_SHIFT and MASK.

The binary values are identical.

Reviewed by: Elliott Mitchell <ehem+freebsd@m5p.com>
Reviewed by: royger
Pull request: #1490

sys/xen/hvm.h

@@ -87,7 +87,7 @@ enum {
 };
 #define HVM_CALLBACK_VECTOR(vector) \
     (((uint64_t)HVM_CB_TYPE_VECTOR << HVM_CB_TYPE_SHIFT) \
-   | (((vector) & HVM_CB_GSI_GSI_MASK) << HVM_CB_GSI_GSI_SHIFT))
+   | (((vector) & HVM_CB_VECTOR_VECTOR_MASK) << HVM_CB_VECTOR_VECTOR_SHIFT))
 
 enum xen_hvm_init_type {
 	XEN_HVM_INIT_EARLY,